Roles of the chest wall and diaphragm in respiratory mechanics

This chapter is most relevant to Section F1(iii) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "describe the structure of the chest wall and diaphragm and to relate these to respiratory mechanics". This appears to be a high-yield topic which comes up multiple times in the exam:

• Question 21 from the second paper of 2015 (anatomy of the diaphragm and its function in respiration)
• Question 2 from the first paper of 2011 (anatomy of the diaphragm and its function in respiration)
• Question 14 from the second paper of 2010 (muscles of respiration and their function)

Thus far the past papers have been largely musculocentric and no questions regarding the involvement of the skeleton have been asked, but it seems relevant to discuss it here anyway. The relevance of the bones in respiratory function becomes suddenly more apparent when they break and start flailing unproductively.

In summary:

In terms of published resources, the official college textbooks are surprisingly effective. Chapter 56 from the 8^th edition of Nunns contains enough material to easily pass the abovelisted SAQs. For a free alternative, one may easily use Luce & Culver (1982) - they cover the same ground, but with more detail regarding the effects of lung pathology on respiratory muscle use.  For just the chest wall muscles, one could not do better than De Troyer et al (2005). For the diaphragm, Poole et al (1997) is best, though it is paywalled. 

Role of the upper airway muscles in respiration

Without recapitulating the content of the upper airway anatomy chapter, it will suffice to say that the upper airway muscles are at least as important as the diaphragm and chest wall because they maintain airway patency during inspiration. Certainly, the examiners felt strongly enough about this that in their marking of Question 14 from the second paper of 2010 candidates who omitted this aspect "failed to achieve a good score".

In summary:

• As the airway pressure becomes negative, this soft muscular tube may collapse
• Airway dilator muscles maintain airway patency during inspiration by contracting
• This is a reflex (pharyngeal dilator reflex) which is mediated by stretch receptors in the upper airway, via the trigeminal, superior laryngeal and hypoglossal nerves
• Of the muscles involved, most of the heavy lifting (literally) is done by the genioglossus, which elevates the hyoid and base of the tongue.  
• On expiration, the thyroarytenoid muscles adduct the vocal cords to increase upper airway resistance and promote a sort of "auto PEEP"

Anatomy of the diaphragm

Question 21 from the second paper of 2015  and the very similar Question 2 from the first paper of 2011 asked trainees to "outline the principal anatomical features of the diaphragm that are important to its function" and to "outline the anatomy of the diaphragm / describe the function of the diaphragm in respiration".  As the latter is a better example of good question design, this section will focus on answering it in detail (whereas the former implies that there may be principal anatomical features which are not important to the function of the diaphragm).

Anatomy of the diaphragm can be described in a boring fashion, following the normal pattern used by anatomy textbooks. Most of this information is from Last's (9^th ed, p. 248-251). In summary:

• Basic structural anatomy: 
  • Thin sheet of skeletal muscle, oval in shape, composed of a central  noncontractile tendon and two discrete muscular portions, the costal and crural diaphragm.
  • From the circumference, fibres arch upwards into a pair of domes and then descend to a central tendon which has no bony attachment.
  • The right dome is higher than the left
  • The central tendon is at the level of the xiphisternum
• Relations: 
  • Superiorly: pericardium and basal lung segments
    (the central tendon is continuous with the pericardium)
  • Inferiorly: 
    • Right: liver, adrenal gland, kidney (the central tendon is also blended with the fibrous capsule of the liver)
    • Left: stomach, adrenal gland, kidney and spleen 
  • Posteriorly: crura (right and left crus), plus 3 arcuate ligaments: median (joins the two crura), medial (a thickening over the psoas), and the lateral (a thickening over the quadratus lumborum)
    • Also aorta, azygos veins, oesophagus, vagus nerve, pleura
  • Anteriorly: tendinous origin is from the  lower six costal cartilages and posterior aspect of the xiphoid process
• Openings in the diaphragm:
  • Aortic opening (at the level of T12)
  • Oesophageal opening (at the level of T10)
  • Vena cava foramen (at the level of T8)
  • Smaller openings for the hemiazygos vein, splanchnic nerves, superior epigastric vessels, lymphatics
• Blood supply: ​​​​​
  • Costal margin supplied by the lower five intercostal and subcostal arteries.
  • Main central mass supplied on their abdominal surface by right and left
    inferior phrenic arteries from the abdominal aorta
  • The phrenic nerve is supplied by the pericardiacophrenic artery 
• Innervation: 
  • Motor: right and left phrenic nerves (C3, 4 and 5, but mainly C4)
  • The lower intercostal nerves send some proprioceptive fibres to
    the periphery of the diaphragm

Anatomical features which are important to its function, if one could isolate those, would probably have to include the following:

• Bilateral nerve supply (i.e. the loss of one phrenic nerve does not adversely affect the function of the entire diaphragm)
• Motor innervation is solely from C3,4 and 5 via the phrenic nerves, which renders it vulnerable to high spinal cord damage
• Slow-twitch fibres (favour sustained activity)
• Circumferential attachment to the costal margin and dome-like shape allows the diaphragm to increase intrathoracic volume by its contraction.
• Nerve supply extends radially from the centre, which has implications for diaphragmatic injuries and lacerations 

Function of the diaphragm

In short, the diaphragm contracts,  flattens, and intrathoracic volume increases thereby. Gauthier et al (1994) performed some 3-dimensional reconstructions of chest CTs and produced some excellent images to illustrate the change in its shape during inspiration:

To be more specific, the diaphragm performs the following movements:

• Downward movement
• Flattening
• Tilting anteroposteriorly (when in the supine position)
• Increase in circumference (this is passive, due to the changes in the dimensions of the ribcage)

The piston-like downward movement accounts for 90% of the volume change seen during normal restful breathing. As you inhale more and more deeply, the flattening of the diaphragm against the counterpressure of abdominal muscles and their contents produces an increase in the circumference of the lower ribcage. 

The diaphragm on its own does most of the work of breathing, but it appears that without the other muscles of respiration working in a coordinated effort, the diaphragm does not achieve the correct pattern of thoracic volume expansion. Danon et al (1979) looked at this in a group of C1 quadriplegics. The diaphragm, when paced independently of other respiratory muscles, did something weird to the ribcage: the lateral diameter increased, but the anteroposterior diameter decreased. In short, the chest flattened and widened on inspiration. "Quiet inspiration is not accomplished by the diaphragm alone but rather by coordinated activity of the diaphragm, rib cage inspiratory muscles, and possibly the abdominal muscles as well" the authors concluded. 

There is a "zone of apposition" which is a posterior portion of the pleural space where there is no lung and the diaphragm is directly apposed with the rib cage. This area, in normal breathing, is surprisingly vast (according to Gauthier et al,  at FRC in the upright position 55% of the diaphragm's surface is apposed with the ribs in this fashion). To again borrow the reconstructions from  Gauthier et al, this can be better explained in a diagram:

This zone of apposition should be viewed as something of a marker of respiratory reserve. With increased inflation (COPD, asthma, etc) the diaphragm, flattened as it is, has less room to move, and its contraction will be less effective.

Apart from acting as a respiratory piston, the diaphragm has important non-respiratory functions:

• Non-respiratory manipulation of thoracic pressure:
  • Coughing
  • Sneezing
• Increase of intra-abdominal pressure:
  • to expel urine or faeces
  • in childbirth
  • during vomiting
• As a mechanical barrier to intra-abdominal organs (keeps the out of the chest cavity) and intra-abdominal fluid (eg. ascites)
• As a functional oesophageal sphincter - the right crus of the diaphragm loops around the oesophageal hiatus; thus when it contracts  it adds pressure to the oesophagus and acts to prevent the reflux of stomach contents when abdominal pressure increases during inspiration

Hiccups is also a diaphragmatic phenomenon, and though one cannot describe it as a "function" with a straight face, it is still worth mentioning.

Chest wall muscles

The anatomy of the chest wall muscles would probably be mainly related to the intercostal group

• Basic structural anatomy: 
  • Two thin layers that span each of the intercostal spaces
  • External intercostals span from tubercles of the ribs dorsally to the costochondral junctions; fibres run in the caudal-ventral direction
  • Internal intercostals  span from the sternocostal junctions to near the tubercles of the ribs; fibres run in the caudal-dorsal direction
  • Anteriorly, the external intercostals are replaced by a fibrous aponeurosis
• Relations: 
  • Superiorly: superior rib and intercostal neurovascular bundle
  • Inferiorly lowe rib
  • Internally: parietal pleura
  • Externally: skin and loose connective tissue
• Blood supply: ​​​​​
  • Anterior and posterior intercostal arteries
  • Costocervical trunk, internal thoracic and musculophrenic arteries
• Innervation: 
  • Motor and sensory: intercostal nerve

In addition to the internal and external intercostals, there are a few more minor players:

•  Levator costae, a small thin muscle joining the upper edge of a rib with the corresponding transverse process of a vertebra. The position of this muscle suggests that it should probably lift the rib (hence the name) but in truth nobody has ever been able to demonstrate any major mechanical disadvantage from its loss. 
• Triangularis sterni (or, transversus thoracis), an internal thoracic muscle which apparently "is almost completely without function". If it had one, it would be largely an expiratory function (it depresses the ribs).
• Scalene muscles which elevate the rib case, counteracting the downward movement of the diaphragm

Their function can be summarised as "expand the chest cavity diameter". This is accomplished by the combination of tow movements:

• "Bucket handle" movement: elevation of the ribs (mainly by the external intercostals)
• "Pump handle" movement: elevation of the sternum (by the sternomastoid muscle)

The net effect of these functions is to increase the diameter of the thoracic cavity (particularly the lower thorax). It is important to emphasise that the inspiratory work is mainly done by the external intercostals, whereas the internal intercostals have a predominantly expiratory role (i.e. by contracting they depress the ribs). Wherever the diaphragm is unable to fulfil its role (eg. bilateral phrenic nerve paralysis or poor excursion due to flattening in COPD) these muscles will be recruited to the role of primary respiratory motors.  And they will suck at this job, as the diaphragm does around 80% of the inspiratory work. Hart et al (2002) found a group of patients with differing degrees of unilateral and bilateral diaphragmatic paralysis and examined their responses to exercise. It turned out that a unilaterally paralysed diaphragm was essentially a non-issue: these patients had a relatively normal response to exercise load. In contrast, patients with bilateral diaphragm paralysis had a lower peak minute volume (60% of what is expected from healthy controls) and used much more oxygen to achieve it, suggesting that their ventilation was inefficient as well as ineffective.

Contribution of abdominal muscles to respiration

The abdominal muscles involved in respiration include:

• Rectus abdominis
• Transvers abdominis
• External and internal obliques
• Pelvic floor muscles 

Montes et al (2016) found that the transversus abdominis and internal oblique seems to be the most relevant muscles for breathing mechanics. These maintain intra-abdominal pressure and act during expiration to push the diaphragm back up into the chest, augmenting the passive recoil of the lungs. Fortunately, most of the time these muscles are not relaxed anyway - for instance, they are contracting constantly to help maintain posture. As such, there is always some positive intraabdominal pressure to help stuff the relaxing diaphragm back up into the chest. 

These muscles are probably either minor players or completely inactive in normal quiet breathing.  Moreover, the effects of posture seem to matter. When supine, abdominal contents is encouraged into the chest by gravity, and so abdominal muscles are not usually needed. In the upright position, they may be required with relatively modest respiratory effort. 

Again according to a number produced by Nunn's with no reference, their forceful expiratory activity doesn't really appear until one's minute volume is around 40L/min. This probably comes from studies such as Goldman et al (1987), who stabbed up to 24 EMG needles into his patients to record abdominal muscle activity at different respiratory efforts. Abdominal muscle contraction activity was first detected at a minute volume of 102L/min for supine patients and 88L/min for semirecumbent ones (40° head up).

Accessory muscles of respiration

These extrathoracic muscles are recruited to assist respiratory effort when the energy requirements of ventilation are increased, whether because the requirement for minute ventilation is increased or because it is requiring more effort to achieve a normal minute volume (eg. where lung compliance has deteriorated).

• Sternocleidomastoid
• Trapezius
• Pectoralis group
• Extensors of the vertebral column
• Serratus anterior
• Latissimus dorsi

Variably, the scalene muscles are group with this bunch, even though some authors group them together with the intercostals as they are also involved in quiet low-effort respiration (Campbell, 1958). 

So, how much effort is required before one resorts to using these muscle groups?  Nunn's gives a figure of 50L/min as the roundabout figure, though it is not clear where that comes from. As effort increases, scalene activity increases first, then the sternocleidomastoids, then the others.